The present invention relates generally to optical light sources and, more particularly, to super luminescent diodes (SLDs) having reduced internal reflectivity.
Optical technologies associated with sensing, instrumentation, and communication have evolved significantly over the last several decades. For example, over the last two decades optical communication technologies have transition ed from laboratory curiosities to mainstream products which are the fundamental means for high speed/high bandwidth communications, advanced sensors, and high precision instruments. Various light sources can be used in these diverse applications. Lasers, for example, can be used to generate constant wave (CW) or pulsed optical signals suitable for use in communication devices. Incas/IMP quantum well lasers with suitable guiding and gain regions can be driven to generate optical pulses for transmitting data in fiber optic communication networks. The optical output of a laser, however, may not be suitable for all optical applications. Low coherence interferometry and coherence domain reflectometry are examples of optical applications in which it is preferable to use light sources having a high power output over a much broader bandwidth than is generally available using lasers.
Super luminescent diodes (SLDs), like lasers, use stimulated emission as a primary mechanism for generating light, but are not intended to exceed the threshold for laser oscillation. Even though lasing is not intended in SLDs, various internal reflections occur within conventional SLDs which may result in spectral output variations which are undesirable in a number of applications, including those mentioned above, and which may (under adverse conditions) result in lasing. To better understand these undesirable reflections, consider the exemplary SLD device 10 illustrated in FIGS. 1(a)–(b).
Therein, a generalized, side section of an SLD device 10 is shown in FIG. 1(a) including a contact region 12, a first cladding layer 14, an active region 16 which establishes a vertical waveguide and a second cladding layer 18. Waveguide 16 confines the optical energy to the active region 16 in the vertical dimension in FIG. 1(a). Pumping current I is injected into the contact region 12 to pump active region 16 to generate light via spontaneous emission. Active region 16 can, for example, be fabricated as a multiple quantum well structure (e.g., alternating layers of GaInAs or GaInAsP) or a bulk active region (e.g., GaInAsP or AlInGaAs). The active region can also contain additional layers (e.g. GaInAsP or AlInGaAs) to form a separate confinement heterostructure (SCH) to tailor the optical waveguide properties of active region 16. First and second cladding layers 14 and 18, which can be fabricated from IMP, operate to contain carriers vertically within the active region due to their higher bandgaps. Light generated within the waveguide created by the layers 14–18 is output via a front facet 20 in the direction indicated by the output arrow. Typically, the optical output energy would be coupled to, e.g., an optical fiber (not shown) to be guided to another device.
Reflections can occur when light generated by injection luminescence strikes an interface between the front facet 20 of the SLD 10 and the outside environment (e.g., air). At normal incidence to the facet, the magnitude of the reflected optical energy will primarily depend upon the phase index n of waveguide 16 of the SLD device 10 relative to the phase index of the ambient. Taking for example the ambient to be air (n1=np,air@STP@589nm=1.00029) and the SLED waveguide index to be n2=np,SLED=3.2, a typical value for the amount of power reflected at the normal incidence facet is approximately 27% (R=(n2−n1)2/(n2+n1)2). These reflections, symbolized by the arrow RFF in FIG. 1(a), will travel back along the waveguide created by layers 14–18 to the back facet 22 of the SLD, where they will again be reflected as indicated by the arrow RBF. These reflections may themselves be captured by the waveguide and transmitted as an unintended double-pass amplified output of SLD 10. This results in various undesirable effects including, for example, the possibility that downstream devices will treat the reflections as optical “echoes” of the power that is intentionally transmitted by the SLD 10. Another undesirable effect which may occur, particularly at high output powers, is that the reflections which are being generated back and forth between the front facet 20 and back facet 22 may result in a spectral modulation or “ripple” on the optical energy output from SLD 10. This spectral ripple causes ghosting in low coherence interferometry applications, makes it difficult to provide power control over the optical power generated by SLD 10 and, in many cases, renders the SLD useless if the magnitude of the spectral ripple is too high. Many applications, for example, require the peak-to-peak spectral ripple to be less than 0.5 dB. The mean reflectivity at each facet (R) required for a SLD with internal single pass gain (G) to obtain a desired spectral ripple (ΔG) is given by R=G−1(ΔG−1)/(ΔG+1). This means that for a high-power SLD, which can have a single-pass optical gain of 30 dB or more, the mean power reflected at each facet must be less than 0.006%. The need for this extremely low facet reflectivity represents one of the major challenges in producing high-performance, high-power SLDs.
Various techniques have been employed in an attempt to reduce internal reflections in SLDs. One technique, seen in the FIG. 1(a) is to apply antireflective (AR) coatings 24 and 26 to the front facet 20 and back facet 22, respectively. However, AR coatings providing less than 0.1% power reflectivity are extremely difficult to manufacture. AR coatings are also not broadband, i.e. they exhibit a wavelength dependent reflectance spectrum. Thus, while applying an AR coating can be effective within a certain narrow bandwidth, it may not be a viable solution for SLDs that generate wider bandwidth spectra.
Another technique, described in an article by A. T. Seminov, V. R. Shidlovski and S. A. Safin, entitled “Wide spectrum single quantum well superluminesent diodes at 0.8 um with bent optical waveguide”, Electron. Lett, vol. 29, pp. 854–857, 1993), is to provide an angle θ at the interface between the lateral waveguide 17 and the front facet 20. This technique is, for example, illustrated in FIG. 1(b) which figure is a top sectional view taken along section line A—A in FIG. 1(a). As used throughout this specification, such “angles” are referenced with respect to facet normal. The vertical waveguide 16 and lateral waveguide 17 form a 2-dimensional waveguide with a defined optical mode that propagates along the length of the chip. As one skilled in the art is aware, the lateral waveguide 17 can be formed by various means for example a ridge waveguide or a buried waveguide. Although the angled facet can reduce the reflections that propagate back through the waveguide to the back facet 22, and has the advantage that the effective facet reflectivity is inherently broadband, nonetheless some reflections will still occur. The amount of reflection can be further reduced by using a combination of AR coatings and angled facet, as shown in FIG. 1(b). Yet even with this combination it can be difficult to obtain the required low facet reflectivity for high-power, high-performance broadband SLDs.
Another mechanism that has been proposed for dealing with internal reflections is to dump the rearward traveling reflections into an absorbing region 17 indicated in FIG. 1(b). The absorbing region 17 is formed by not injecting current along the entire length of the active waveguide 16, thereby creating an unpumped section 13 indicated in FIGS. 1(a) and 1(b). Since this section is not pumped, it absorbs light rather than emits light thereby attenuating the rearward traveling reflections and preventing them from reaching the back facet 22. The main disadvantage of this approach is that it results in a longer chip which translates directly into higher production cost. This is particularly the case for quantum-well active regions which have a small confinement factor and are readily “bleached” at high optical power. As shown in the article by Song, J. H. et al., entitled “High-power broad-band super luminescent diode with low spectral modulation at 1.5-μm wavelength”, Photonics Technology Letters, IEEE, Volume: 12, Issue: 7, Jul. 2000, Pages: 783–785, good spectral ripple can be obtained using an absorber, but at the expense of more than doubling the chip length.
To reduce the length of the absorbing region, it has also been proposed to provide an active (reverse biased) absorption region to absorb reflections in the region proximate the back facet 22, as shown in the side view in FIG. 2(a) (wherein the reference numerals are reused from FIG. 1(a) to denote similar structures). Therein, a second contact region 24 is placed over the portion of the SLD 26 proximate the back facet 22. The contact region 22 is separated from the contact region 12 to create an absorption region and a gain region, respectively, within the SLD 26. The contact region 22 is reverse biased relative to the contact region 12 using a voltage VB. This has the effect of biasing the absorber region which increases the amount of absorption and also increases the upper wavelength at which absorption occurs above that of the emission wavelengths of the gain region, thereby acting to further absorb reflections traveling along the waveguide/active region 16. However, this solution suffers from the potential drawback that the interface between the absorber region and the gain region will itself cause reflections. Also, a second electrode that requires a separate bias voltage introduces additional complexity that increases the overall cost of operating the SLD. An example of a reverse-biased absorption region can be found in U.S. Pat. No. 5,252,839, the disclosure of which is incorporate here by reference.
Yet another approach to reduce facet reflections is to fabricate so-called “window” sections adjacent to the facets. Using exemplary window sections 28 and 29 in FIG. 2(a), the optical mode is no longer confined and is allowed to freely diffract such that reflections from the facet do not couple as efficiently back into the waveguide as compared to SLDs wherein the waveguide extends all the way to the facet. However, these window regions are of limited utility because they cannot be made too long otherwise the freely diffracting beam can no longer be captured effectively with practical optical elements. Also, the window regions require complicated wafer fabrication processes to selectively remove the active region and replace it with epitaxial regrowth of the upper cladding material. The incremental reduction in facet reflectivity achieved with window structures often does not warrant the added fabrication complexity and associated yield loss/cost increase.
Accordingly, it would be desirable to provide SLD techniques and devices that provide high power and high quality output optical energy by reducing internal reflections.